Piezoelectric actuators are used extensively in communications equipment, in high precision measurement devices, and in many other areas. In particular, they are often used in magnetic head and disk testers (see, for example, U.S. Pat. No. 6,006,614) for data storage devices.
Conventional piezoelectric actuators for those applications typically include an electrostrictive element disposed between electrically conductive drive electrodes, all within an electrically insulating envelope, or shell. Often conventional piezoelectric actuators include a lead carbonate titanate (PZT) material disposed adjacent to insulating lead oxide in grain boundaries of the lead carbonate titanate material.
It is well known that piezoelectric actuators are sensitive to a significant extent to external agents. The presence of environmental moisture, for example, is known to cause electrochemical ion migration on the surface of the electrostrictive element of an actuator, leading to a deterioration or corrosion of insulation characteristics within the actuator. This moisture-based phenomenon is exacerbated by contamination by electrically conductive materials. In actuators including lead carbonate titanate (PZT) material, the same result often occurs due to reduction of the insulating lead oxide in the grain boundaries of the lead carbonate titanate (PZT) material. Such corrosion of piezoelectric actuator components are known to affect dynamic features of the actuator.
The practice of working with magnetic head and disk testers shows that the impact of the environment on piezoelectric actuators is often responsible for significant deterioration of operational parameters of such testers over time, and in particular in relation to the precision of head positioning with respect to a disk under test.
It is known in the art, to protect a piezoelectric actuator from certain aspects of ambient surroundings using hermetic encapsulation. A number of methods for so protecting piezoelectric actuators by encapsulation have been proposed in U.S. Pat. No. 4,803,393, U.S. Pat. No. 5,113,108, U.S. Pat. No. 7,665,445, U.S. Pat. No. 8,193,686, and others. In essence, those proposals are different embodiments of one basic approach depicted in FIG. 1.
In FIG. 1, an encapsulated piezoelectric actuator 100 principally comprises an electrostrictive element 101 and an elongated envelope 102. The elongated envelope 102 includes a correspondingly elongated inward-facing wall extending about and along a central displacement axis CDA, defining a correspondingly elongated interior chamber. The interior chamber extends from a proximal end P of envelope 102 to a distal end D of envelope 102. As illustrated, envelope 102 is an elongated (in the direction of axis CDA) corrugated structure, having periodic variations in radius along the axis CDA.
A proximal end element 103 spans the proximal end P of envelope 102, sealed at its periphery to the inward-facing wall of envelope 102, for example, by welding. A plate 104 spans the distal end D of envelope 102, sealed at its periphery to the inward-facing wall of envelope 102, for example, by welding. Together, the weld junctions of both proximal end element 103 at the proximal end P of envelope 102, and plate 104 at the distal end D of envelope 102, with the inward-facing wall of envelope 102, establish the interior chamber as hermetically sealed.
The electrostrictive element 101 is disposed within the interior chamber, extending about and along axis CDA, from proximal end P of envelope 102 to distal end D of envelope 102. Lateral surfaces of the electrostrictive element 101 are spaced apart from the inward-facing wall of envelope 102. Electrostrictive element 101, at its proximal end, is affixed to an inward-facing surface of proximal end element 103, and, at its distal end, is affixed to an inward-facing surface of plate 104.
Two electrical leads 105 pass through glass seals in the proximal end element 103. Within the interior chamber, distal ends of leads 105 are connected to a pair of electrically conductive elements 106 extending along opposite lateral surfaces of electrostrictive element 101. A hermetically sealable gas inlet 107 passes through the proximal end element 103. That inlet is used to selectively evacuate air from the envelope 102 and/or to fill it with a gas as desired.
As noted, the envelope 102 is an elongated (in the direction of axis CDA) corrugated structure, including exemplary corrugation 108, so that the inward-facing wall of envelope 102 has variations in radius in along the axis CDA. The corrugated structure establishes, in effect, a series of extendible (in the direction of axis CDA) flexures, enabling envelope 102 to expand or contract in length in a direction along the central displacement axis CDA of the piezoelectric actuator, tracking expansion and contraction of the electrostrictive element 101 along axis CDA, without breaking the gas-tightness of the envelope 102.
While the structure of the prior art actuator of FIG. 1 can, to a point, generally perform certain of the desired functions needed in magnetic head and disk testers for example, there are important limitations of such structures. For example, hermetic sealing of proximal end element 104 to the envelope 102 requires welding, or a similar difficult to perform and costly operation, of the periphery of proximal end element 103, and plate 104, to envelope 102. Further, maintenance of such actuators in an operational setting, typically requires periodic replacement of the electrostrictive element 101. With the structure of FIG. 1, opening of the hermetically sealed interior chamber of envelope 102 (typically sealed by welding or the like) is required, with a following re-sealing (typically re-sealed by re-welding or the like). In view of these difficulties and shortcomings of the prior art, improved encapsulated piezoelectric actuators are needed.